1. Field of the Invention
The present invention relates to an industrial method for purifying acetic acid, particularly to a method for purifying acetic acid obtained by carbonylation of methanol.
2. Description of the Related Art
Acetic acid is a basic chemical employed as a starting material of each of, for example, acetic esters, acetic anhydride, vinyl acetate and terephthalic acid and thus employed in large quantity in the petrochemical, organic chemistry, pharmaceutical and agricultural chemical production, polymer chemistry and other industries.
Various industrial processes are known for producing acetic acid, which include, for example, oxidation of acetaldehyde, direct oxidations of hydrocarbons such as petroleum naphtha and butane, and carbonylation of methanol. Of these, the process in which acetic acid is produced by continuously carbonylating methanol with carbon monoxide (see Japanese Patent Publication-B No. 47-3334) is now widely employed as an industrial process for producing acetic acid.
In recent years, reaction conditions and catalyst improvements have been studied in the above acetic acid production through carbonylation of methanol. For example, a process in which a catalyst stabilizer such as an iodide salt is-added to the reaction system has been disclosed see G.B. Patent Publication-A No. 2,146,637 (published on Apr. 24, 1985)!. In Japanese Patent Publication-A No. 60-54334, it has been proposed to cause a relatively large amount of iodide ions to be present in a reaction fluid so as to lower the concentration of water in the reaction fluid, thereby minimizing the energy required to obtain purified dry acetic acid per unit quantity thereof and further omitting a purification process for obtaining acetic acid as a product see European Patent Publication-A No. 573,189 (published on Dec. 8, 1993)!.
Although the acetic acid produced by the carbonylation of methanol has a relatively high purity, it is known that the acetic acid contains a minute amount of reductive impurities as evaluated by the potassium permanganate test conducted in accordance with JIS K 1351 see European Patent Publication-A No. 322,215 (published on Jun. 28, 1989)!. The potassium permanganate test is an important industrial test which acetic acid must pass when it is put in various practical uses as a product. Therefore, the removal of the above impurities leading to poor results in this test is of utmost importance.
The conventional purification methods for obtaining commercially suitable high-quality acetic acid capable of meeting the standards include, for example, (a) chemical addition method for example, addition of an amine, a nonvolatile acid or the like as disclosed in Japanese Patent Publication-B No. 42-6243 (published on Mar. 14, 1967)! and (b) oxidant addition method for example, addition of peracetic acid as disclosed in Japanese Patent Publication-B No. 48-30615 (published on Sep. 21, 1973)!. However, the above methods require the use of various additives, so that problems arise with respect to handling and aftertreatment. That is, not only is at least simple distillation for separating the additives further required but also there is a potential problem of the additives remaining in the acetic acid as a product. Especially, the oxidant addition method (b) has a potential danger of explosion attributed to a rapid reaction of peroxide remaining unreacted in separating the additives. Therefore, a purification method in which no additives are added is desired.
While, in a distilling method, concentration of a minute amount of impurities to a considerable extent is required for minimizing acetic acid entrained in the separation and disposal of impurities to thereby suppress the purification loss of the acetic acid. Thus, the distillation column must have a considerably large number of plates and a considerably high reflux ratio. Consequently, a considerably large amount of thermal energy is to be consumed, thereby bringing about a problem of gravely pushing up the cost for purification. Therefore, a method of economically removing the above impurities is desired.